A vehicle antitheft system for preventing theft of a vehicle is proposed in Japanese Patent Unexamined Publication No. H08-40206, for example. This system includes an immobilizer unit incorporated in a vehicle, and a portable unit. Into a ROM provided on the immobilizer unit side, an encryption key, a password, and an ID identical to those stored in the portable unit are written, using equipment in a vehicle manufacturer's factory or a dealer. After this setting, authentication is performed between the immobilizer unit and the portable unit with a high level of security. When the immobilizer unit authenticates the portable unit, the engine of the vehicle is started.
In such a system, authentication is performed with a high level of security. However, special equipment is necessary to write the encryption key, the password, and the ID identical to those stored in the portable unit, into the ROM in the immobilizer unit.